1. Field of the Invention
The present invention relates to an electrode low in contact resistance to a p-type Group III nitride compound semiconductor. It also relates to a method for producing an electrode with reduced contact resistance. Incidentally, the concept “Group III nitride compound semiconductors” includes semiconductors represented by the general formula: AlxGayIn1-x-yN (0≦x≦1, 0≦y≦1, 0≦x+y≦1) which includes binary compounds such as AlN, GaN and InN, ternary compounds such as AlxGa1−xN, AlxIn1-xN and GaxIn1-xN (0<x<1 each), and quaternary compounds such as AlxGayIn1-x-yN (0<x<1, 0<y<1, 0<x+y<1).
2. Description of the Related Art
When, for example, Group III nitride compound semiconductors are applied to light-emitting devices, the Group III nitride compound semiconductors are direct transition type semiconductors having a wide emission spectrum range of from ultraviolet to red. The Group III nitride compound semiconductors are applied to light-emitting diodes (LEDs), laser diodes (LDs) and so on. Because each Group III nitride compound semiconductor has a wide band gap, there is an expectation that devices using Group III nitride compound semiconductors will operate more stably at a high temperature than devices using other semiconductors. For this reason, the application of Group III nitride compound semiconductors to transistors such as FETs has been developed actively. In addition, because each Group III nitride compound semiconductor contains no arsenic (As) as a main component, there is an expectation that Group III nitride compound semiconductors will be developed to various semiconductor devices for general purposes from an environmental aspect.
In a compound semiconductor, it is usually impossible to obtain an ohmic contact when metal is formed simply on a surface of the semiconductor. Therefore, after a metal film is formed on the compound semiconductor, alloying is made by a heat treatment (sintering) to diffuse metal into the semiconductor to there by obtain an ohmic contact. Particularly in a p-type Group III nitride compound semiconductor, even in the case where resistance is reduced by a heat treatment such as electron beam irradiation, the resistivity of the p-type Group III nitride compound semiconductor is still higher than the resistivity of an n-type Group III nitride compound semiconductor. Accordingly, for example, in a light-emitting device, there is little lateral spread of a current in the p-type layer, so that light is emitted only just under the electrode. Therefore, a current diffusing electrode formed in such a manner that films of nickel (Ni) and gold (Au) each having a thickness of the order of tens of nm are laminated and heat-treated has been proposed as an electrode having both light-transmitting characteristic and ohmic characteristic (e.g., see Patent Document 1). Even in this case, when, for example, the electrode for p-type gallium nitride (GaN) is composed of Ni and Au, the contact resistivity pc of the electrode is high to be 7×10−3/cm2 in the present circumstances.
Incidentally, as described above, in the compound semiconductor, it is impossible to obtain an ohmic junction when metal is formed simply on the semiconductor surface, so that simple deposition of metal generally brings Schottky characteristic. It is conceived that this is because a Schottky barrier ΦB (ΦB=(X+Eg)−Φm) to movement of a carrier is formed in a boundary between the compound semiconductor and the metal as shown in FIG. 1 (e.g., see Non-Patent Document 1).
On the other hand, oxide and other deposits are present on a surface of the compound semiconductor, so that these deposits cause increase in contact resistance between the compound semiconductor and the metal. To solve this problem, for example, a chemical treatment and a sputtering treatment using an inert gas have been proposed (e.g., see Patent Document 2).
[Patent Document 1]
Unexamined Japanese Patent Publication No. Hei-6-314822
[Patent Document 2]
Unexamined Japanese Patent Publication No. Hei-8-264478
[Non-Patent Document 1]
Tetsuji Imai et al., “Compound Semiconductor Devices [II]”, Kogyo Chosakai Publishing Co., Ltd., pp.73–76
It is said that reduction in the Schottky barrier ΦB or the width of a depletion layer formed in the boundary between the p-type semiconductor and the metal is indispensable for reduction in contact resistance between the compound semiconductor and the metal and further leads to greater reduction in contact resistivity at the time of formation of an ohmic junction based on a heat treatment after that. In the present circumstances, essential issues such as the substance of the compound semiconductor/metal boundary, the relation with the metal boundary structure, and so on, are not yet exactly known. It is an urgent necessity to settle the essential issues.